(1) Field of the Invention
This invention relates to the recovery of waste heat, and more particularly to an apparatus for the recovery of heat from high temperature gas stream.
(2) Description of the Prior Art
Cogeneration and heat recovery systems normally recover heat from hot gases leaving a gas turbine, gas engine, or other source of hot gas by generating steam which is used for process or space heating, for driving a steam turbine and generating electricity or for driving a pump or compressor. Infrequently, a high temperature hot oil stream is used to recover heat and transfer the heat to other streams via heat exchangers.
A frequent application of heat recovery systems is one which hot gases leaving a gas turbine in the range of 800.degree. to 950.degree. F. produces power at a steady rate which is sold to a local power company, split between the power company and operating company or used by the operating company. Generally, there is an excess of heat available or the heat uses vary depending on plant operation, and therefore, some of the hot gases by-pass the heat recovery system via expensive and somewhat unreliable large by-pass stack damper. An expensive and complex control system is required to automatically modulate the opening of such damper, and in the case of failure of the by-pass system, steam pressure will rise to elevated pressures and/or the heat transfer oil will rise to an elevated temperature requiring automatic shutdown of the gas turbine or the source of the hot gases.
In a hot oil recovery unit, heat transfer design is very complex and expensive because oil will break down if its maximum film temperature (730.degree. to 800.degree. F.) is exceeded due to oil flow interruptions, gas flow interruptions or overfiring. Therefore, the capital cost is as high as is operating cost to replace or replenish the heat transfer oil.
In U.S. Pat. No. 4,257,579 to Bruhn et al. and assigned to the same assignee as the present invention, there is disclosed a waste heat recovery system using at least two heat exchange recovery systems utilizing intermediate heat transfer mediums for a process operation producing an exhaust or waste gas at temperatures of from 500.degree. to 2500.degree. F. wherein the heat exchanger assemblies may be fabricated using conventional materials of construction and wherein heat may be recovered at levels substantially higher than with the use of a single intermediate heat transfer medium, however, such systems depend upon using two or more heat recovery systems and each system has to have the ability to fully use all the heat that is recovered in each system.